Not Applicable.
1. Field of Invention
The present invention relates generally to power amplifiers and, more particularly, to circuits and methods for suppression of noise and spurious signals in power amplifiers systems.
2. Description of the Background
In communications systems using electromagnetic radiation, the reception quality of the received signal is related to the signal to noise ratio (SNR), i.e., the ratio of the signal strength to the noise level at the receiver. Typically, the amount of noise introduced by the power amplifier at the transmitter is negligible compared to the thermal noise at the receiver. This is especially true for long range applications, such as on the order of hundreds of miles or more. Consequently, it is ordinarily not necessary to utilize noise suppression techniques at the transmitter. Rather, to improve the SNR, it is often sufficient to merely increase signal power at the transmitter.
However, for short range applications, the amount of noise introduced at the transmitter becomes increasingly important. This is because the noise introduced by the transmitter may approach, or even exceed, the thermal noise floor at the receiver. Moreover, for narrow band applications, such as with radar systems, the SNR cannot be improved merely by increasing the signal power from the transmitter because of the non-linearity of the power amplifier at saturation, which may cause intolerable spectral regrowth (mostly in pulse modulated radar systems). Moreover, because noise is random, pre-distortion techniques cannot be used.
Accordingly, there exists a need for a manner to suppress noise and other spurious signals in a power amplifier. There further exists a need for a manner to inexpensively suppress noise and other spurious signals in a cascaded amplifier system.
The present invention is directed to an amplifier system. According to one embodiment, the amplifier system includes a vacuum tube amplifier having an input signal terminal and an output signal terminal, and a phase noise suppression circuit having first and second input terminals and an output terminal, wherein the first input terminal is coupled to the input signal terminal of the vacuum tube amplifier, the second input terminal is coupled to the output signal terminal of the vacuum tube amplifier, and the output terminal is coupled to one of an electron source of the vacuum tube amplifier and an interaction region of the vacuum tube amplifier.
According to another embodiment, the amplifier system includes a vacuum tube amplifier having an input signal terminal, an output signal terminal, and an electron source, and an amplitude noise suppression circuit having first and second input terminals and an output terminal, wherein the first input terminal is coupled to the input signal terminal of the vacuum tube amplifier, the second input terminal is coupled to the output signal terminal of the vacuum tube amplifier, and the output terminal is coupled to the electron source of the vacuum tube amplifier.
According to another embodiment, the amplifier system includes a first amplifier having an input signal terminal and an output signal terminal, a second amplifier having an input signal terminal and an output signal terminal, wherein the input signal terminal is coupled to the output signal terminal of the first amplifier, and a phase noise suppression circuit having first and second input terminals and an output terminal, wherein the first input terminal is coupled to the input signal terminal of the first amplifier, the second input terminal is coupled to the output signal terminal of the second amplifier, and the output terminal is coupled to the first amplifier.
According to yet another embodiment, the amplifier system includes a first amplifier having an input signal terminal and an output signal terminal, a second amplifier having an input signal terminal and an output signal terminal, wherein the input signal terminal is coupled to the output signal terminal of the first amplifier, and an amplitude noise suppression circuit having first and second input terminals and an output terminal, wherein the first input terminal is coupled to the input signal terminal of the first amplifier, the second input terminal is coupled to the output signal terminal of the second amplifier, and the output terminal is coupled to the first amplifier.
In contrast to prior techniques, the present invention provides an efficient and inexpensive technique for suppressing noise and other spurious signals for vacuum tube amplifiers. Moreover, the techniques of the present invention are applicable for amplifier systems having two or more cascaded amplifiers. These and other benefits of the present invention will be apparent from the detailed description of the invention hereinbelow.